1. Field of the Invention
The present invention relates to a network connection system which is capable of offering special services such as a three way calling service and so forth in an electronic telephone exchange of the time division system type or the space division system type using electronic contacts such as transistors or the like.
2. Description of the Prior Art
A variety of special services can easily be offered through utilization of an electronic exchange and they can be combined in various ways. Combining all the special services involves an enormous computer program, and hence is not practical. Accordingly, it is general practice in the prior art to offer a limited number of several kinds of special services or some limited number of combinations of special services.
In a conventional electronic exchange having mechanical contacts, release or the like of a subscriber in an intra-office call is supervised by an intra-office trunk circuit and a special service connection to the subscriber can be accomplished by connecting the subscriber within the local office system via the intra-office trunk circuit. When the special service connection is accomplished, by regarding the intra-office trunk circuit as a combination of an outgoing and an incoming trunk, that is, by processing the intra-office connection as a combintion of an outgoing and an incoming connection, the necessity of using a specialized program for the intra-office connection can be eliminated, and consequently the program can be simplified.
In an electronic exchange of the time division system type or the space division system type which employs electronic contacts such as transistors or the like, because a DC loop cannot be supervised through such a switching network, it is impossible to provide an intra-office trunk equivalent to that used in the electronic exchange having mechanical contacts. Accordingly, an intra-office connection cannot be handled in the same manner as in the electronic exchange provided with mechanical contacts, and a complicated program is required for offering various special services when the contacts are electronic.